Synthetic slag, sometimes referred to as "flux" or "mold powder", has been used in the continuous casting of metals including aluminum- or silicon-killed steels, austenite stainless steels, etc., for several purposes. Firstly slag is used to protect the molten metal from air oxidation. Constituents of the melt, such as iron, aluminum, manganese, chromium, titanium and the like are converted to oxides when exposed to air. These oxides are a source of non-metallic inclusions in the solidified metal the presence of which is decidedly undesirable. At the same time, the loss of these constituents through oxidative processes changes the compositional makeup of the resultant alloy. Secondly, the mold powders are used as fluxes to solubilize and remove oxide impurities present in the melt, therefore allowing for the production of clean, inclusion-free steel. The mold powders are further used to improve the lubrication between the mold and the solidifying strand of steel as it is continuously withdrawn.
In the trade the terms "flux" and "slag" often have been used interchangeably to describe fritted, or preponderantly fritted, mold powders which are used in the continuous casting industry. In the present context, "flux" is defined as a fluorine-bearing frit or compound that can be used to lower the viscosity of molten slag compositions at high temperatures.
During continuous casting of steel, increasing amounts of oxide impurities present in the molten steel are absorbed into the molten slagging composition. With increasing service time, the performance of the slagging composition deteriorates to such a degree that the steel output of the caster must be slowed down because the molten composition cannot transfer heat away from the forming solid steel shell fast enough to thicken the shell sufficiently. Also, the surface of the steel being cast shows more and more inclusions because the molten slagging composition cannot absorb impurities. At the same time, with the increasing presence of oxide impurities in the slagging composition, its viscosity may increase to such a high value that its necessary function as a lubricant for the mold is no longer provided. The rise in the viscosity of the liquid slagging composition may hinder its movement into the gap between the mold wall and the forming solid steel shell. In such instances where the gap is not lubricated due to the absence of liquid slag, the steel shell may seize on the wall of the mold and the likelihood of breakout can not be discounted. When one or more of these occurs, corrective actions must be taken and the caster run must be interrupted or shut down completely.
In the art of glass making, it has long been known that the incorporation of fluorine effectively reduces the viscosity of glass compositions at high temperatures. It could not however been projected that the incorporation of fluorine in the glass composition would be beneficial in the context of flux in continuous casting applications.
The atmosphere in the upper segment of a steel caster is usually one of high temperature and moisture. Fluorine released from the mold powder may cause metallic corrosion in the upper segment of the caster as a result of acid formation. Additionally, for most continuous casting processes it is important that little or no water be introduced with the flux. The presence of even small amounts of absorbed moisture (e.g. 5+% relative to the weight of flux) in the context of casting of steel may cause excessive oxidation. Accordingly, the dryness of the flux is preferably maintained and more preferably, the flux is made to contain no hygroscopic components.
Relevant synthetic slag compositions for continuous casting of steel have been disclosed in U.S. Pat. No. 3,649,249. The composition thus disclosed is said to have a high solubility of aluminum oxide. The slags thus disclosed containing high amounts of fluorine and at least some lithium, an element the absence of which characterizes the present invention. Also relevant to the background of the present invention are U.S. Pat. No. 3,704,744 which disclosed synthetic slag composition containing fluorine and lithium having a high solubility for aluminum oxide, and U.S. Pat. Nos. 3,899,324 and 3,926,246 which disclosed flux compositions which contain as much as 15% fluorine. Lastly, U.S. Pat. No. 4,290,809 disclosed a raw mix flux for the continuous casting of steel. The chemical analysis of the mixture show the raw batch to contain as much as 25% fluorine.
It is an objective of the present invention to provide a vitrified glass containing a large amount of fluorine. It is another objective of the present invention to provide a vitrified glass suitable for blending with other components to prepare mold powders for applications in continuous casting of steels and other metals.
It is another objective of the present invention to provide a process for the production of high fluorine frit and fritted particulates.